ESTRO 2021 Abstract Book

S311

ESTRO 2021

Purpose or Objective Radiation therapy (RT) dose escalation may improve clinical outcomes for patients with inoperable pancreas cancer (PCa), although can potentially cause severe toxicity due to the proximity of gastrointestinal luminal organs. A few studies have suggested that significant dose escalation may be safely achieved using magnetic resonance image (MRI) guidance with on-table adaptive replanning although long-term outcomes are not well understood Materials and Methods A single institution retrospective analysis was performed of 50 consecutive patients with inoperable non- metastatic pancreas adenocarcinoma treated on a 0.35T-MR Linac without fiducial markers from 2018- 2020. Median age was 70 years (range 35-91). ECOG performance status was mostly 0-1 (94%). Locally advanced disease was most common (82%), otherwise borderline resectable (8%) or medically inoperable (10%). Median CA19-9 at diagnosis was 122.2 U/mL (range, 0.9-12,868.6). Induction chemotherapy (CT) was routine (94%), usually FOLFIRINOX (58%) or gemcitabine/nab-paclitaxel (26%), for a median 4.2 months (range, 0.2-14.5). Local control (LC) was defined using RECIST 1.1 criteria. Acute (<3 months from SABR) and late toxicity were assessed as per CTCAE v5 criteria. Outcomes were assessed from the date of PCa diagnosis. Results Median prescribed dose was 50 Gy (range 40-50), median biologically effective dose (BED 10 ) was 100 Gy 10 . The median PTV volume was 111.0 cm 3 (range 11.3-374.33). Treatment in breath hold was common (90%). On- table adaptive replanning was performed for a median 5 fractions (range 1-5) in 96% of patients. Post-SABR therapy included pancreaticoduodenectomy (14%), irreversible electroporation (10%), and/or CT (52%). Negative margins were achieved in 6/7 patients who had surgery. Median follow-up was 18 months (range 7-39 months) from diagnosis. Median, 1-yr, and 2-yr estimated LC was not reached, 97.8%, and 88.9%, respectively. Median, 1-yr, and 2-yr estimated progression free survival (PFS) was 16 months, 67.7%, and 25.7%, respectively. Median, 1-yr, and 2-yr estimated overall survival (OS) was 21 months, 87.9%, and 50%, respectively. Induction CT duration >3 months was the only predictor of improved PFS (hazard ratio [HR] 0.358; 95% confidence interval [CI] 0.141-0.913; P=0.031) and OS (HR 0.288; 95% CI 0.104-0.799; P=0.017) on multivariate analysis. Acute and late grade 3+ toxicity rates were 2% and 10%, respectively. Conclusion To our knowledge, this is the largest series of 5-fraction MRI-guided stereotactic ablative body radiation therapy (SABR) with on-table adaptive replanning for inoperable PCa. Our 2-yr estimated OS of 50% is significantly higher compared to historical outcomes of ~20% from CT +/- non-ablative RT. Additional study is needed to improve our understanding about optimal patient selection, treatment response assessment, and the need for additional therapy after SABR. OC-0416 Results of SBRT-SG-01, a prospective multicentric-study of SBRT for liver metastases. M.C. Rubio Rodríguez 1 , F. Arias 2 , X. Chen-Zhao 3 , S. Flamarique 2 , P. Fernández-Letón 4 , M. Campo 2 , M. López 5 , M. Rodríguez 2 , D. Zucca 6 , D. Martínez 7 , E. Sánchez-Saugar 5 , F. Mañeru 8 , J. García Ruiz-Zorrilla 9 , P. García de Acilu 9 , J. Valero 1 , A. Montero 1 , R. Ciérvide 1 , B. Alvarez 1 , M. García-Aranda 1 , R. Alonso 3 , M.A. de la Casa 4 , L. Alonso 4 , M. Nuñez 1 , J. Martí 4 , O. Hernando 1 1 Hospital Universitario HM Sanchinarro. HM Hospitales. , Radiation Oncology, Madrid, Spain; 2 Complejo Hospitalario de Navarra, Radiation Oncology, Navarra, Spain; 3 Hospital Universitario HM Puerta del Sur. HM Hospitales. , Radiation Oncology, Madrid, Spain; 4 Hospital Universitario HM Sanchinarro. HM Hospitales. , Medical Physics, Madrid, Spain; 5 Hospital Universitario HM Sanchinarro. HM Hospitales, Radiation Oncology, Madrid, Spain; 6 Hospital Universitario HM Sanchinarro. HM Hospitales, Medical Physis, Madrid, Spain; 7 Complejo Hospitalario de Navarra, Medical Physics, Navarra, Spain; 8 Complejo Hospitalario de Navarra, Medical Physcis, Navarra, Spain; 9 Hospital Universitario HM Puerta del Sur. HM Hospitales. , Medical Physics, Madrid, Spain Purpose or Objective To evaluate the efficacy and tolerability of high-dose stereotactic body radiation therapy (SBRT) for the treatment of patients with liver oligo-metastases. Primary end point was local control. Secondary end points were toxicity and survival. Materials and Methods Patients (p) with 5 or less liver metastases, <7cm diameter, were enrolled and treated on this multi- institutional prospective study. Patients included had un-resectable liver metastases, were inoperable o had rejected surgery. ECOG 0–1 and adequate liver function was required.